1. Field of Invention
This invention relates to the recovery of volatile organic compounds (VOC) vapors for reuse or disposal in an environmentally safe manner.
2. Description of Prior Art
The release of volatile organic compounds (VOCs), especially hydrocarbons (HCs) into the atmosphere has been found to cause an increase in the ozone content of the lower atmosphere. Ozone is formed in the air as a result of photochemical reactions when HCs (such as gasoline vapors, paint fumes, or dry-cleaning fumes from solvents) combine with nitrogen oxides, oxygen, and sunlight. Ozone is a product of weather conditions, yet current knowledge of atmospheric chemistry is very limited. There is a seasonal pattern to VOC emissions, since a marked increase is VOCs occurs in the summer months as the heat causes gasoline and other hydrocarbon liquids to evaporate more quickly. At high concentrations, ozone can adversely affect human health, agricultural crops, forests, and other materials. See "Controlling Hydrocarbon Emissions from Tank Vessel Loading," Committee on Control and Recovery of Hydrocarbon Vapors from Ships and Barges, Marine Board Commission on Engineering and Technical Systems, National Research Council [hereinafter "Controlling Hydrocarbon Emissions from Tank Vessel Loading"], Appendix C at 177-178 (1987).
As a result of the harmful effects of ozone, the Environmental Protection Agency, (EPA), has established a primary ozone level standard to protect public health of 0.12 ppm (1-hour average) or 235 micrograms per cubic meter not to be exceeded more than one day per year. Currently the EPA is reviewing available scientific and technical information to determine whether the standard is adequate to protect human health and welfare. Some evidence suggests that even attainment of the existing standard for ozone will not protect public health with an adequate margin of safety. Id.
Because the states must devise State Implementation Plans (SIPs) to provide for the attainment of the NAAQS for ozone, the states are now searching for VOC sources where emissions can be reduced. Several states are now targeting the more difficult to control or smaller sources and thus are proposing that ships or barges that load VOCs be fitted with vapor control equipment. Id. at 194.
The cost of emission control is a central issue. Case studies conducted at the direction of the Committee on Control and Recovery of Hydrocarbon Vapors from Ships and Barges suggest that installing an operating vapor control facility, in order to achieve the maximum allowable limit of 3 lb of vapor emitted per 1000 barrels of product transferred, at a small terminal in Texas would add $0.008 per gallon of gasoline loaded while the cost at a larger terminal would increase costs by $0.0036 per gallon. Some smaller companies, especially in the inland barge industry, may have problems financing the necessary investments. Calculations confirm the strong dependence of cost effectiveness on terminal throughput. Id. at 108.
In addition to the cost problem, there are technical problems because available vapor recovery units are not operable or available for use in marine environments where terminals are space constrained and have safety constraints. For instance, the Coast Guard will not permit the use of a flare on a barge so that incineration of vapors produced while loading from an offshore marine terminal onto a boat would not be allowed. Moreover, in this circumstance, the available large stationary vapor recovery units would be expensive to install on the marine terminal and expensive to operate at low levels of utilization. Other government agencies such as the Office of Safety and Health Administration (OSHA) also have regulations which are directed to the safety of operating personnel and which constrain the operation of a conventional VRU under these conditions.
Hydrocarbon emissions also constitute a loss of product so that there may be an economic incentive to recover the hydrocarbon vapors if the cost of recovery is lower than the value of the product lost. These economics therefore depend upon the cost of equipment, the level of utilization of the capital equipment, the cost of operating the equipment, labor costs, etc., and the market value of the recovered vapors.
Current vapor control technology may be divided into three categories: (1) closed loading of tank vessels, more properly termed vapor balancing; (2) incineration; and (3) recovery processes.
Closed loading of tank vessels necessitates loading with all the hatches and ports closed. This is contrary to most barge practice but is routine on most large tank ships. It is noteworthy that the term "closed loading" does not necessarily imply the capture of vapors, rather, as a tank is being filled, the vapor in the free space above the level of the liquid being loaded is displaced upward into a pipeline which returns the vapor to the free space of the tank being emptied. Thus, the vapor is in effect recycled from the tank filling up to the tank being emptied.
Combustion or incineration processes are more than 98% efficient if operated properly. They can perform reliably as the sole hydrocarbon control process but even more reliably as polishing units. The primary drawback is that they do not recover the hydrocarbon product. The value of this incinerated hydrocarbon can be significant when crude or gasoline is being shipped. Furthermore, combustion devices can be relatively unsafe because they are potential sources of ignition for the flammable VOCs and hydrocarbon products. It is also noteworthy that the incineration process produces NO.sub.x which contribute to smog. Thus, incineration is to an extent a self-defeating method since it contributes to the very ill that is being sought to be eliminated.
Vapor recovery processes may be divided into three types: (1) lean oil absorption; (2) refrigeration; and (3) carbon bed absorption. Id. at 71. Lean oil absorbers operating at pressures of 100 to 200 psia are very efficient at recovering hydrocarbons from rich streams but less efficient at removing hydrocarbons from streams that contain little hydrocarbon. Typically, an absorber can remove up to about 95% of the ethane and heavier fraction of the vaporous hydrocarbon content of a feed stream by pressure increase and temperature decrease. At temperatures below 60.degree. F., hydrate formation may cause freeze-up problems. If the system is under pressure, water can also freeze at temperatures above 32.degree. F. Antifreeze can be used to lower the liquid hydrocarbon freezing point but this adds to operating costs. The absorption process can only reduce a vapor stream's hydrocarbon content to 1-3% (volume) of the initial ethane and heavier fraction economically. Thus, the absorber off gas should be routed to a polishing flare or incinerator. Id. at 72-73.
The direct refrigeration system removes hydrocarbons by cooling and condensing the vapors through a series of low temperature heat exchanges. This process has the advantage that very low temperatures are possible so that up to 99% of a stream's hydrocarbon content can be removed. However, in order to achieve this high proportion of hydrocarbon reduction, temperatures below 60.degree. F. may be required and at these temperatures hydrates may form and plug the exchanger surfaces and lines. This can be avoided by the injection of ethylene glycol or other antifreezes. Even the best DRUs which employ vapor compression and expansion with regenerative heat exchange against very cold expander discharge refrigerants cannot remove ethane and heavier hydrocarbons to the very low levels required by regulatory authorities, i.e., three pounds of hydrocarbon vapor emitted per 1000 barrels loaded. The obvious solution would be to incinerate this stream in a flare, however, the use of such flares are a safety hazard and are unacceptable to the Coast Guard authorities for use on board a ship. Moreover, flares produce NO.sub.x and are to that extent counterproductive since NO.sub.x contributes to smog. Further, the DRU exit stream is so lean that hydrocarbon would have to be added to enrich it to enable combustion. This is a waste of product which was costly to recover in the DRU process. The safe, efficient disposal of lean light hydrocarbon vapor streams remains a problem.
Carbon bed absorbers use activated carbon or a similar absorptive material to absorb hydrocarbons selectively. After the absorptive capacity of the medium is used up, the hydrocarbon will "break through" and appear in increasing amounts in the exiting vapor stream. At this point, the medium is recharged. The spent carbon may be disposed of but if the volume is large enough, regeneration of the carbon can be cost effective. The best approach is to use a vacuum to desorb the hydrocarbon from the carbon. As an alternative, the hydrocarbon can be steam stripped from the carbon but this generates an oily waste water stream that has to be disposed of. Carbon beds do not do a good Job of recovering light ends such as ethane and propane. For use in marine applications, carbon beds would need to be very large to handle the high flow rates and hydrocarbon loadings generated.
It is noteworthy that while in the oil and chemical industry hydrocarbon vapors are recovered from streams that are essentially "steady state," in loading operation, the hydrocarbon composition and vapor quantity is not steady state. For example, at commencement of loading a crude oil the initial vapor is low in quantity and may consist largely of light hydrocarbons. As loading progresses, the quantity of the vapor increases and heavier hydrocarbons are also present in the vapor. Thus, a vapor recovery system for these operations must be able to cope with an unsteady state vapor stream.
The above technology is large and capital intensive. In order to obtain a return on the investment, a high level of utilization is necessary. These technologies may therefore be useful at large terminals where there is a high throughput of hydrocarbons and other products which produce VOCs during the loading and off-loading processes.
There are, however, a large number of smaller or remove terminals which do not have a large throughput of product and which also would produce VOCs during loading or off-loading processes albeit in small quantities but which nevertheless contribute to the overall VOC emissions. Among these are smaller land-based terminals and offshore oil production rigs. It would be prohibitively expensive to construct vapor recovery units on each offshore oil producing platform or at each small on-land terminal to recover the relatively smaller amounts of VOCs produced by each such source even though in sum they may produce an appreciable tonnage of VOCs.